Magnetoresistive Random Access Memory (MRAM) is a non-volatile memory technology that uses magnetic elements. For example, Spin Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) uses electrons that become spin-polarized as the electrons pass through a thin film (spin filter). STT-MRAM is also known as Spin Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
Referring to FIG. 1, a diagram of a conventional STT-MRAM cell 100 is illustrated. The STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ) storage element 105, transistor 110, bit line 120 and word line 130. The MTJ storage element is formed, for example, from a pinned layer and a free layer, each of which can hold a magnetic field, separated by an insulating (tunnel barrier) layer as illustrated in FIG. 1. The STT-MRAM bit cell 100 also includes a source line 140, sense amplifier 150, read/write circuitry 160 and bit line reference 170. The operation and construction of the memory cell 100 is known in the art and will not be discussed in detail herein. Additional details are provided, for example, in M. Hosomi, et al., A Novel Nonvolatile Memory with Spin Transfer Torque Magnetoresistive Magnetization Switching: Spin-RAM, proceedings of IEDM conference (2005), which is incorporated herein by reference in its entirety.
As shown in FIG. 1, the STT-MRAM cell 100 has a source line 140 that is connected via a pad (not shown) to the lower portion of the transistor 110. Conventional pad designs use large metal grid layers (arrays) or large metal plates which cover the entire pad area which lead to large capacitance from the probing pads, and therefore, lead to signal distortion and sometimes even lead to signal extinguishing particularly for short pulse signals or high frequency signals.